The electrical characteristics of devices which compose an integrated circuit (IC) must be controlled appropriately for normal operation of the IC. However, impurity contamination from the outside and improper process conditions often cause the degradation of device characteristics. As for the metal oxide semiconductor (MOS) IC, a large number of reports have been published on the degradation phenomena and mechanism of the electrical properties of MOS field effect transistors (FETs); that is, (i) gate oxide breakdown failure due to poor dielectric strength, 1 (ii) drain leakage current failure due to carriers which are generated by a generation-recombination center 2 such as metal impurities and crystal defects, (iii) threshold voltage (V th ) instability due to short channel effect and narrow channel effect, 3 (iv) V th instability due to penetration of boron (in a boron-doped p ϩ -polysilicon gate electrode) into the channel region through the gate oxide, 4-6 (v) V th instability due to alkali ions such as sodium incorporated into gate oxide, fixed oxide charge formed very near the Si/SiO 2 interface in oxidation process step, and oxide trapped charge due to ionizing radiation damage, 3 and (iv) degradation of transconductance due to surface state located at the Si/SiO 2 interface. 3 In this paper, we report a new degradation mode for the threshold voltage instability due to the conductivity change of the channel region in an n-channel MOSFET (NMOSFET).
ExperimentalNMOSFETs with polysilicon gates were fabricated on 6-8 ⍀ cm, 125 mm diam, (100) boron-doped p-type silicon substrates. The initial oxygen concentration, as determined by Fourier transform infrared (FTIR) spectroscopy analysis, was about 1.4 ϫ 10 18 cm Ϫ3 . An intrinsic gettering treatment with three-step temperatures (1200 r 650 r 1000ЊC) 7 was used in pad oxidation for local oxidation of silicon isolation. The field oxide was grown at 1000ЊC in a pyrogenic stream. After removal of the pad oxide, 90 nm of gate oxide was grown at 1000ЊC in a 3%HCl/O 2 ambient. Then, boron was implanted through the gate oxide at 40 keV with a dose of 4.5 ϫ 10 11 cm Ϫ2 for the threshold voltage adjustment, followed by a low pressure chemical vapor deposition of undoped polysilicon. Then, phosphorus was implanted into the polysilicon gate at 100 keV with a dose of 6 ϫ 10 15 cm Ϫ2 , followed by an activation annealing in N 2 at 1000ЊC for 30 min. After polysilicon patterning and etching, As ions were implanted at 40 keV with a dose of 4 ϫ 10 15 cm Ϫ2 , followed by an activation annealing in dry O 2 at 1000ЊC for 20 min to form an n ϩ source/drain junction. After borophosphosilicate glass (BPSG) depositon and densification in N 2 at 1000ЊC for 40 min, contacts were defined, followed by Al metallization and sintering in forming gas at 450ЊC for 15 min. The passivation film is a phosphosilicate glass (PSG). Figure 1 shows the cross section of devices used for evaluations; i.e., active NMOSFET, polysilicon gate field MOSFET, MOS capacitor, and n ϩ -p junction diode. The gate ...
